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Pinning down the Milky Way’s spinBY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 24, 2008

Using ESO’s HARPS instrument, astronomers studying pulsating stars – cepheids – have determined the rotation of our Galaxy, and shown that it is much simpler than previously thought.

Cepheids have been used as distance indicators since their discovery by Henrietta Leavitt in 1912, thanks to the Period-Luminosity relation, which relates the variability period of a Cepheid to its absolute mean magnitude. The longer a Cepheid’s pulsation period, the more luminous the star, and since measuring a Cepheid’s period is easy, the period-luminosity relation allows astronomers to determine the Cepheid’s brightness and hence its distance. If the Cepheid is in another galaxy, the Cepheid’s distance gives the distance to the entire galaxy. By looking at their velocities, Cepheids have also been used to infer the rotation of the Milky Way, but neighbourhood cepheids show an apparent ‘fall’ of towards our Sun, an observation that has been much debated.

"The motion of Milky Way Cepheids is confusing and has led to disagreement among researchers," says Nicolas Nardetto, lead author of a paper that describes the findings, soon to be featured in the journal Astronomy and Astrophysics. "If the rotation of the Galaxy is taken into account, the Cepheids appear to 'fall' towards the Sun with a mean velocity of about two kilometres per second."

Artist's impression of the local neighbourhood of the Sun and its setting within our galaxy, the Milky Way (see insert above). The figure shows the positions of some bright stars (in white) in the sky as well as the eight Cepheids used in the investigation (in blue). After the rotation of the Milky Way had been accounted for (red arrow), it seemed that the Cepheids were all 'falling' towards the Sun (blue arrows). New, very precise measurements with the HARPS instrument have shown that this apparent 'fall' is due to effects within the Cepheids themselves and is not related to the way the Milky Way rotates. The motion indicated by the blue arrows is thus an illusion. Image: ESO.

A debate has raged for decades as to whether this phenomenon was truly related to the actual motion of the Cepheids and consequently to a complicated rotating pattern of our Galaxy, or if it was the result of effects within the atmospheres of the Cepheids themselves. Nardetto and colleagues observed eight Cepheids with the high precision HARPS (High Accuracy Radial Velocity Planetary Searcher) spectrograph, attached to the 3.6 metre ESO telescope at La Silla in Chile. HARPS is usually associated with planet hunting, but it also lends itself to determine precise radial velocities, that is, the speed with which something is moving towards or away from us.

“The problem of the apparent motion of Cepheids has occurred for almost a century now. The last result in this field used a non-axi-symmetric model of the rotation of the Milky Way,” Nardetto tells Astronmoy Now. “We show that the rotation of the Milky Way is actually simply axi-symmetric. There are no non-axi-symmetric complex motions as described in previous work. Our observations show that this apparent motion towards us almost certainly stems from an intrinsic property of Cepheids.”

The astronomers found that the deviations in the measured velocity of Cepheids were linked to the chemical elements in the atmospheres of the Cepheids considered. “We demonstrated that there is a correlation between the apparent motion of Cepheids and the spectral line forming regions considered,” Nardetto explains. “We found a new way to calibrate physically the apparent motion of Cepheids which has never been done before, that's why our result is extremely strong.”

However, the research team confess that they still don't know what is happening in the atmospheres of the Cepheids. “This velocity term could be due to permanent mass loss, limb-darkening effect within the spectral line, velocity gradient within the line-forming region, a relative motion of the line-forming region with respect to the corresponding mass elements (which means that it is not always the same part of the atmosphere which absorbs the light coming from the photosphere), and, most likely, a combination of all these effects,” speculates Nardetto.

Nardetto concludes that although they find that the rotation of the Milky Way is simpler, the dynamical structure of Cepheids' atmospheres is more complex than previously expected, which could have strong implications on their use in determining distance scales in the Universe.

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